Proper AC gauge readings on retrofitted 300ZX

[Big Dawg]

My 1990 300ZX was retro fitted to accept R134a refrigerant. Looking at the Nissan Online Service Manual it appears to suggest a low side pressure of 24 to 29 psi at 86 degrees ambient temps and a high side of 164 to 201 psi. But these readings were established when the system had R12.
Are the same readings appropriate for a system that is now running R134a? If not then what would be the correct readings at different temps?

[Nacho]

High pressure could be 10-15% higher than that. Low pressure should be below 30 at idle.

[Big Dawg]

Yikes!
Unless I did not give the car and ac sufficient time to get to operating temps, my low side was 45psi and the high side was around 255 psi at about 1,000 rpm. The high side would rise to close to 280 psi at 1,500rpms.
It was approx 85 degrees F in the garage (I used an infrared non contact laser thermometer), although I didn't measure the ambient temps near the car engine (always been confused where the ambient temps should be taken). My vent temps were very good...38 to 42 degrees F.
The evacuation, retro fit and installation of the R134a was done at a AC repair shop. That's all they do. Is this seriously overcharged???

[dave1182]

I wouldn't necessarly say over charged, its hard to tell if you didn't put the refrigant in yourself or have written proof of what was put in, the orginal charge of R-12 calls for 34 ounces, and R-134a should be about 28 ounces, and the pressures sound about right, best way to check that i have found is multiply 2.2 xambent temp, but in all honsty I would wait for Nacho or Tom, AcProf to chime in. Remember those 300zx use a mechanical fan and at curb idle the cooling effect can drive you Nuts at times and i have found a electric fan helps a lot.

Good Luck.

[Nacho]

How much gas was put into the system?

[Big Dawg]

According to the work order, if I am reading this right... 26 OZ of R134a and 5 OZ of Pag.

I did a recheck ( I let the car get to operating temps and then let the AC run till the car was cool before opening the low and high side coupling valves) and the low side is now showing 40psi and the high side is showing a steady 250psi.

Also, I had replaced the AC fan a long time ago with an electric fan controlled by radiator temps and when the compressor turns on or off.

[Nacho]

My favorite number for most retrofits is 24 ounces. This week we did a couple of Grand Marquis and we are just delivering a 1990 Jeep Cherokee with only 24 ounces.
We came up with that number based in pressures and duct temperature. In my opinion, there is nothing written as to the ideal percentage of R12 chrage. We've found this percentage to be 65% (Grand Marquis) to 70% on the Cherokee.

I'd give your 300ZX a try with 22-24 ounces.

[Probedude]

Why is R134A so unforgiving of overcharge?

If the condenser is big enough, won't all the 'overcharge' just turn to liquid at the temp that the condenser is at?

[Nacho]

I don't think your condenser is big enough to handle ANY overcharge. The specs of a systems go beyond a condenser size. Like I said, is a matter of fine tunning a lower charge (65-70%) than the old spec, 26-4 - 29.92 ounces. Try 20-22 ounces.

[Big Dawg]

Is 2 oz. more or less really going to cause my Z to be over charged?
And is my 40 psi low and 250 psi high with in reason for my car?

[ACProf]

I did a recheck ( I let the car get to operating temps and then let the AC run till the car was cool before opening the low and high side coupling valves) and the low side is now showing 40psi and the high side is showing a steady 250psi.

Well, 40 psi puts your evaporator core temp at 45 degrees and 250 psi puts the condenser at 145 degrees. Giving 5-10 degrees transfer temp that puts the evaporator vent air at 55 degrees. Not too good, if the car interior was cooled down. Condensers with adequate airflow through them should run 30-40 degrees above the incoming ambient air temperature (Rule of thumb: multiply the ambient air temp X 2.2 to get the correct high side pressure). So if the outside air was 105 degrees when you took the 250 psi (145 degrees) measurement, the high side is working to spec.

If it was LESS than 105 degrees out at the time, then the condenser temp is probably up due to inadequate air flow and/or overcharge and the excess heat load generated by this excess refrigerant in the high side of the system is flowing back with the refrigerant into the low side, preventing adequate heat absorption and evaporation action in the evaporator, causing higher low side pressure and temperature, resulting in poor performance.

You be the judge.

[Big Dawg]

ACProf:
Thanks for the detailed info.
I removed a very small amount of freon (maybe an oz.) from the system just to see how the low and high sides would change if at all.
Here are the findings:
Windows up, doors closed, box fan on high at front of the car, 1,500rpms, hood up. in garage with garage door open and 93 degrees outside temps.
Waited 10 minutes then shut car off and attached low and high side gauges.
started car and waited till gauges settled in.
Low side is now 30 to 32 psi
high side is still between 248 and 250psi.
Vent temps at the center console is 52 to 53 degrees using digital thermometer and 48 to 52 degrees using infrared non contact thermometer.
Even when my Z was new it never really cooled great on very hot days...wasn't till the 1991 models came out that the Japanese rectified the problem. I'm told that R134a will not cool quite as well as R12, so I wonder if the vent temps are as good as I'm going to get or if there is still room for improvement.
By the way...I also tried the above with doors closed and windows opened (still used box fan) and found virtually no difference in readings.